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Date: Sat, 24 Nov 2018 17:08:15 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: No 2200m TX tonight
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Hello Paul,

Ooh it is a class-E PA? That explains much! They need a very accurate 
SWR to reach the high efficiency. Initially i've been a friend of 
class-E but what you see now is exactly the reason why i'm preferring 
class-D now!
Generally: Avoid TO-220 cases for a real PA. Prefer TO-247 instead, 
especially below 500 kHz.

Regarding the effect you're observing: I would guess it is a thermal 
problem in a ferrite core, a capacitor or even the away-drifting 
on-resistance / working point of the class-E PA.

73, Stefan

Am 24.11.2018 12:47, schrieb N1BUG:
> Hi Markus,
>
> That is very interesting. It changes about 25 % during the first 2-3
> minutes, then it seems to settle down and not change any more. It
> could be moisture somewhere, but any moisture here is solid ice or
> frost now. I did not find any ice or frost in the transformer box or
> the variometer. Could be insulators or something with the antenna
> itself. I don't see any "fuzz" on the scomematch voltage trace so I
> think (hope) nothing is arcing.
>
> What worried me is this did not happen last winter so something has
> changed. Everything accumulates some dirt here because of blowing
> dust, smoke, etc. I wonder if a small amount of dirt on insulators
> plus moisture can combine to make funny things happen.
>
> Anyway I examined the little PA and it seems to have died due to
> poor thermal interface between FET and heatsink. It's a physically
> small FET and maybe was not screwed down tight enough with the
> Sil-Pad interface, which was also a previously used one.
>
> Normally I do not like to put drain voltage on the heatsink but as
> an experiment for this little PA (which is totally an experiment
> itself, but served me very well last winter) I will isolate the heat
> sink from the chassis/PCB and mount the little FET directly to it.
> The thermal resistance would be much lower! I think this is fine so
> long as nothing shorts the heat sink to ground. In that case some
> fuses die. ;-) This would add some pf of capacitance between drain
> and ground but it would be in parallel with the quite large C of the
> Class E tank, probably not much difference at 137 kHz!
>
> Parts to repair the big PA should arrive Wednesday.
>
> 73,
> Paul
>
>
> On 11/23/18 4:29 PM, Markus Vester wrote:
>    
>> Hi Paul,
>>
>> sorry to read that. Have pity on the poor FETs!
>>
>> You mentioned that the antenna resistance is gradually decreasing
>> (i.e. improving) during longer transmissions. I often see that
>> effect here, with the current rising by say 20 % during the first
>> few minutes. I've put it down to moisture or dew around the coil
>> and insulators (tiny little polycarbomnnate pencil tubes), which
>> evaporates as things warm up. It is more prominent during cold
>> damp weather, and much more so with the very high antenna
>> impedance at VLF than at LF.
>>
>> Good luck, Markus
>>
>>
>> -----Ursprüngliche Mitteilung----- Von: N1BUG<paul@n1bug.com>
>> An: rsgb_lf_group@blacksheep.org<rsgb_lf_group@blacksheep.org>
>> Verschickt: Fr, 23. Nov. 2018 22:12 Betreff: LF: No 2200m TX
>> tonight
>>
>> No transmissions from me this night. The little amplifier has
>> died. I think it may be related to this resistance change in the
>> antenna which is getting worse and worse.
>>
>> I'm going back to MF for this night, sorry!
>>
>> 73, Paul
>>      
>